to just remove it for standard builds
I'm not sure there is currently a way with gnustep-make, if so i missed it in the documentation.

something like:
DEBUG_OBJCFLAGS=-Werror

so it only occurrs during make debug=yes,
I'm not sure i'm entirely on board with this (that is i see the debian bug you provide has -g), that said it also has optimization flags, which I thought debug=yes disabled.

so they may be passing -g in manually rather than using make debug=yes.

a lot of projects also will only enable it if maintainer mode has been enabled with ./configure --enable-maintainer-mode

or if the version is checked out from version control (e.g. a .git dir exists) as this is removed from the release tarball.

Philosophically speaking, I'm quite in favour of -Werror (no warnings means no petty little problems to obscure big ones), but we don't do it anywhere else (as far as I'm aware), so it might make sense to remove it?